This invention relates to tubeless radial-ply tires for trucks or buses which comprise carcass plies made of an organic fiber and are adapted to be mounted on a frusto-conical bead rim. More particularly, it relates to an improvement relating to flat crown type tires (low profile tires).
A radial ply tire for heavy-duty vehicles such as trucks or buses whose carcass ply cord is made of an organic fiber, e.g. polyamide, polyester or the like has many advantages over the same type of tires comprising a carcass made of steel cords, for example, in respect of reduction of tire weight, economy in fuel cost of the vehicle, fatigue resistance and renewability of the ply cord material (cf. fretting in the case of steel cord), etc. On the other hand, however, the tire has an insufficient stiffness of the carcass plies, particularly in their turned-up portion at the bead portion because of the low flexing stiffness of the cord per se, so that the turned-up portion of the carcass plies and its vicinity are, when the tire is loaded, likely to cause a separation owing to deformation stress.
In order to make up for this insufficiency or deficiency of stiffness, various measures are usually adopted; for example, reinforcement plies of metallic cords are provided at the bead portion, a hard rubber is used, the thickness of the bead portion is increased, etc. These measures, however, produce adverse results concurrently with the results as intended. That is to say, a separation is likely to occur because of a stiffness gap at the end of the reinforcement plies, and the region extending from the bead portion to the sidewall portion is caused an increase of the weight due to the excess volume and a significant energy loss due to heat build-up.
Furthermore, disadvantageous is the case of a tubeless tire to be mounted on a frusto-conical seat rim (for example a taper of 15.degree.), where the breadth of bead base portion is made wider in order to maintain the air-tightness and to secure the portion to the rim. As a consequence, the bead portion has a larger thickness in comparison with a general tube tire (a flat base and a rim of a taper of 5.degree.) while the sidewall portion having a flexibility is of a narrow area, and deformation sometimes occurs in the neighborhood of the bead portion when the tire is imposed with a load. Particularly, with a low-profile tire (e.g., wide base 70-, 75-, 80-series) whose section height is lower as compared with a tire of the same load capacity (H/W=ca. 88%), it is disadvantageous that the sidewall range tends to be narrow.
In general, with a tubeless tire to be mounted on a frusto-conical seat rim, the region from the sidewall portion to the bead portion is shaped and profiled so as to be gradually decreased in thickness from the bead toward the sidewall, at a location of which having its minimum thickness the section width of the tire is its maximum. As a consequence, a region having a small thickness and a flexibility of the sidewall portion tends to be narrow.
When the tire of this kind undergoes a load and bends, its deformation region is near the bead portion, where the end of reinforcement plies and the turned-up end of carcass plies have large deformation stresses. In contrast, in the case of a tire comprising a sidewall portion having a wide range of thin, flexible region, the sidewall portion shares in majority of load deformations and the bead portion is deformed in a small rangae. This is because the foregoing tire comprising the sidewall portion of which thin and flexible region is narrow flexes longitudinally, when subjected to the same load as in the case of the tire having a wide thin-sidewall region, almost equally to the latter tire, so that the region of a large thickness extending from the sidewall portion to the bead portion is forced to be deformed.
Particularly, with a radial ply tire wherein carcass ply cords are disposed at an angle of about 0.degree. relative to the radial direction, it is essential to enhance stiffness and to make the sidewall portion mainly share in the flexure deformations when the tire is loaded, thereby to ensure tire durability. This is performed by either providing a stiff belt ply at the tread portion and a reinforcing ply in the turned-up portion of the carcass plies at the bead portion or increasing a thickness of the bead portion.
In the case of a radial tire adapted to be mounted on a frusto-conical seat rim as specified in this invention, it is still more necessary to render a thickness of the bead portion substantial, thereby to assure a durability since its rim flange height is relatively low and the end of carcass plies wrapped around the bead wires and the end of reinforcement plies are located higher than the rim flange.
In view of the situations described above, various approaches have been taken to make a thin-sidewall region as wide as possible and to ensure a sufficient stiffness for the bead portion.
For instance, an improvement in such tubeless radial tire to be mounted on a frusto-conical seat has been proposed and disclosed in Japanese Published Application (Unexamined) No. 55-19685 (1980). According to it, a tire is shaped to be defined on its outer periphery with at least one recessed portion having a radial length of 30 to 40% of the radial distance between the bead seat and the maximum section width point, when the tire is mounted on a standard rim, inflated and unloaded. Here, the region of the recessed portion usually corresponds to a bead-to-sidewall transitional area which shares in the deformations caused under a load by reason of a continuous change from a stiffness of the bead portion to a suppleness or flexibility of the sidewall portion. Consequently, the deformation strains should be progressively increased from the bead portion to the sidewall portion. However, if there is a local large deformation or bending in that region, it is likely to cause separation or fatigue of the carcass ply cords.
Thus, in the case of this proposed tire, the recessed portion causes the concentration of stresses or discontinuity of strains because of a kind of notch effect, which accelerates the fatigue of that portion and eventually results in the decrease of tire durability.